1. Field of the Invention
The present invention relates generally to the construction of roadway markers and, more particularly, to roadway markers having enhanced pavement adherence capabilities when used with conventional adhesives while exhibiting exceptional strength and economy in manufacture.
2. Description of the Prior Art
Roadway markers have long been used to designate lanes of traffic and roadway centerlines to improve highway safety. Early forms of roadway markers were non-retroreflective and were constructed of a solid piece of ceramic material having a high gloss on its top surface to provide for a degree of specular reflection of incident light and afford a daytime visual indication of roadway lane configurations, for example. These ceramic markers had a generally granular, unglazed bottom surface to improve adhesion and typically were formed with bumped protrusions on their bottom surfaces to improve shear strength in conjunction with suitable adhesive materials.
Such markers were useful only for daytime delineation of lane lines. U.S. Pat. No. 3,332,327, assigned to the common assignee herein, describes a retroreflective marker for retroreflecting incident light from oncoming vehicle headlights, thus providing for exceptional highway marking during nighttime or other low light driving conditions. To provide support for the retroreflective thermoplastic shell and means for attachment to the road, such markers were filled with a thermosetting epoxy and, as with the ceramic daytime markers, were adhered to the road with epoxy adhesives. In early versions of these markers, the bottom surface of the epoxy fill was smooth and glossy; however, it was observed that adhesion of the epoxy filled markers was inferior to that of the ceramic markers and in order to improve adhesion, the bottom of the epoxy was sprinkled with sand to provide the sandpaper-like surface that is still being used today for potted type markers.
In a later version of retroreflective markers, the retroreflective element was molded separately and welded to a thermoplastic body having a waffle-like interior grid of internal cells in order to reduce considerably the amount of material used in manufacture, reduce otherwise prohibitive manufacturing cycles and avoid the undesirable shrinkage that would occur during the molding process if the body were to be molded as a solid piece. An example of such a marker is disclosed in U.S. Pat. No. D-267,933 issued Feb. 14, 1983. Such markers are installed by depositing a suitable adhesive, such as bitumen or epoxy, on either a roadway surface or the underside of the marker base so that the adhesive is sandwiched between the roadway surface and marker, and applying downward pressure to seat the marker in the adhesive. Of the two adhesives mentioned, bitumen has an advantage of rapid setup, enabling marker installation without closing the road to traffic. However, bitumen has greatly inferior adhesive strength compared with epoxy, particularly at elevated ambient temperature, and it was soon noted that the loss rate of waffle-bottom type markers installed with bitumen was, in some instances, much greater than the loss rate of waffle-bottom type markers installed with epoxy or the loss rate of potted markers installed with either adhesive. It is believed that the greater loss rate of the waffle-bottom marker when installed with bitumen is attributable to the adhesive being squeezed out from beneath the narrow cell walls and upward into the cells, thus leaving only a thin film of adhesive or no adhesive between the roadway surface and the downwardly facing surface portions of the marker base.
In order to improve bonding capability of conventional thermoplastic markers alternatives have been proposed to the open-cell narrow wall construction of the marker shell. In one such alternative, the narrow cell walls of the shell are thermally deformed after molding to have generally mushroomed bottom surfaces. This mushrooming of the cell walls serves not only to increase surface area of the cell walls for better bonding, it allows adhesive to flow upwardly into the associated cells and around the mushroomed surfaces of the walls thereby providing a positive mechanical interlocking of the marker to the adhesive. An example of such a marker construction is disclosed in U.S. Pat. No. 5,078,538 issued Jan. 7, 1992 and assigned to the common assignee herein.
While markers as just described improve over earlier construction, they are still not without disadvantages. Specifically, it has been found that markers having internal chambers sealed at their bottoms with adhesive experience pressure differentials between the ambient air pressure acting on the exposed marker shell and the air pressure internal to the chambers. Air pressure within the marker chambers increases notably, for example, if the marker is installed cold and then later is heated by elevated ambient temperatures. This effect of differential pressure reduces the hold-down force on the markers. For example, for a marker of the type manufactured by Stimsonite Corporation as model No. 66 having a cell grid area of approximately ten square inches, the marker will experience an upwardly directed force tending to remove it from the roadway surface of approximately seventeen pounds if the marker is installed at an ambient temperature of 50.degree. F. and later experiences a typical warm weather pavement temperature of 110.degree. F. It can be appreciated that at such an elevated temperature bitumen adhesive will have reduced bonding capability as well. For example, studies have shown that the slant shear strength of bitumen at 100.degree. F. or 120.degree. F. is respectively 15% and 8% of the shear strength at 70.degree.; thus the lifting pressure of trapped air at increased temperatures and the reduced adhesive strength of bitumen at increased temperatures combine to cause a greater loss rate of the waffle-bottom marker adhered with bitumen.
In another alternative to early prior art waffle-like marker shell construction, enhanced surface bonding is achieved by essentially covering the open cell grid of the marker bottom with a perforated plate which is secured to the marker shell as by sonic welding, for example. This construction allows for the surface area of the marker bottom to be maximized while still using the lightweight waffle-like interior construction of the marker shell. An example of such construction is disclosed in co-pending application Ser. No. 08/487,250 filed Jun. 13, 1995 and assigned to the common assignee herein. While such marker construction is particularly effective against the effects of pressure differentials as heretofore described, the use of a grid covering plate adds to the cost of manufacture of the marker by requiring an added part and added steps in the production process.
Accordingly, it is desirable to provide a roadway marker which may readily be produced by known manufacturing techniques as a lightweight but strong and durable product. It is further desirable to provide such a marker which does not exhibit undesirable pressure differentials as would tend to dislodge the marker from the roadway surface under ambient temperature variations present during normal marker use. Still further, it is desirable to provide such a marker having a base surface area which is maximized such that the marker has enhanced adhering properties when installed with a preferred adhesive such as bitumen. Further, it is desirable to provide a marker which is economical to manufacture and exhibits superior quality in manufacture without extraordinary production methods.